1. Field of the Invention
The present invention relates to flowmeters of the vortex shedding type, and more particularly to bluff body configurations for generating vortices in the flowmeter.
2. Description of the Prior Art
While the investigations of development of vortices in flow and the relationship of the frequency of formation of such vortices to the flow rate in a line date back many years, industrial quality vortex flowmeters were first introduced about in 1969. Vortex flowmeters use the phenomena of regular and alternate generation and separation of vortices from opposite sides of a suitably shaped bluff body or bar that is inserted into the fluid stream.
The basis for obtaining accuracy is to insure that the vortices are formed in a stable manner, that is that there aren't any "skips" and that the vortex shedding frequency is in fact proportional to the flow rate past the meter. In describing the vortex shedding behavior of bluff bodies, it is usual to relate the shedding frequency, bar geometry and flow rate using two nondimensional parameters. These are the Strouhal number (S) which is a proportionality constant between the vortex shedding frequency (f), the fluid velocity (v), and the maximum cross sectional width of the bar (H) given by: EQU S=fH/v Equation (1)
and the Reynolds number (R.sub.H) relating the fluid velocity (V), the fluid density (.rho.), the fluid viscosity (.mu.) and the bar width (H) given by: EQU R.sub.H =.rho.vH/.mu. Equation (2)
The bluff bodies or bars that have a constant Strouhal number over a wide range of Reynolds numbers are considered good candidates for vortex flowmeters because their vortex shedding frequency does vary linearly with flow rate.
Vortex flowmeter manufacturers commonly choose cross sections similar to the rectangle, square, triangle or T, since such bodies shed strong vortices. Although these bars shed strong vortices, they must be linearized. Prior art devices have attempted to do this in various ways i.e. by changing the bar width (H) which affects the blockage such bar causes in the conduit or pipe. Linearity of vortex shedding to flow in the conduit remains a primary concern in using vortex shedding flowmeters for geometries that shed strong vortices.
In the prior art many cross sectional variations of bluff bodies or bars have been advanced. One early patent that illustrates a variety of cross sectional geometries for a bluff body flowmeter is the patent to W. G. Bird, U.S. Pat. No. 3,116,639, issued Jan. 7, 1964. The effect of circular cross section bluff bodies mounted ahead of splitter plates or pivoting vanes is shown. Additionally, generally triangular shaped cross sections of bluff bodies and a modified diamond type shape body are shown in FIGS. 10 and 13 of this patent. The sensing of the frequency of vortex formation was done by the use of the downstream, pivoting splitter plate.
U.S. Pat. No. 3,572,117, issued Mar. 23, 1971 to A. E. Rodely illustrates bluff body flowmeters having generally triangular shaped cross sections, as well as variations of the triangular shape. Further, in FIGS. 4C and 6A of this patent, "T" shape cross section bodies are illustrated, and a "cross shaped" cross section also is shown. U.S. Pat. No. 3,572,117 indicates that the upstream facing surface of the body should be flat or convex for increased rangeability.
In U.S. Pat. No. 3,732,731 which is owned by the same company as U.S. Pat. No. 3,572,117, a modified cross sectional shape having a rounded front face is illustrated, and in U.S. Pat. No. 4,069,708 which is also owned by this same company, a plate downstream of the bluff body is used to facilitate sensing of the shed vortices.
U.S. Pat. No. 3,693,438, issued Sept. 26, 1972 to Yamasaki et al. shows a variety of bluff body cross sectional shapes including a cylindrical body that has recesses along a portion of the length of the sides thereof for purposes of enhancing vortex formation. The bluff body response was to be free of the influences of changes in flow and eddy currents in the stream to maintain a linearity of the sensed frequency of the formation of vortices with changing fluid flow. In particular, FIG. 5 of U.S. Pat. No. 3,693,438 shows recessed sides that form a type of a dimple in cross section, while other forms show flat parallel surfaces in the recessed sections.
U.S. Pat. No. 3,948,097 also shows a flow measuring device which uses a rectangular cross section bluff body related in a particular manner to the diameter of the pipe in which it is used and also the patent emphasis that the dimensions of the rectangular cross section should be related to each other for satisfactory operation.
Many of the bluff bodies illustrated in the last two mentioned patents have passageways in the bodies to facilitate the detection of vortices. However, the geometry of the disclosed bluff bodies had to be changed with changing flow line size, and this also influenced the selection of sensors to be used. Thus different sensor construction and size would likely have to be supplied with the bluff bodies for each different size fluid flow pipe.
U.S. Pat. Nos. 3,888,120; 3,946,608; 4,003,251; 4,005,604; and 4,033,189 are typical of the devices placed on the market by Fischer & Porter Co. of Warminster, Pa., and show various bluff body members that have a trailing portion connected to the bluff body through the use of one or more beams or "stings". U.S. Pat. No. 3,888,120 shows various configurations for the upstream bluff body and the trailing rear section in FIGS. 1, 5, 6 and 7 of that patent.
U.S. Pat. No. 4,052,895 which is also owned by Fischer & Porter shows a bluff body having a trailing "tail" assembly connected by an intermediate beam that has a very small cross section and does not extend along the longitudinal axis of the bluff body. Thus flow may interact in the space between the bluff body and the tail.
Additional generally T cross section shapes of bluff bodies and their associated sensors are shown in U.S. Pat. No. 3,972,232. The bluff bodies have head members with facing surfaces and a narrower body section extending downstream from the head member. In this device, the sensor is a member that moves under differential pressures that occur along the side surfaces of the downstream extending sensor bar. This patent discloses the general relationship of positioning of a sensor relative to an upstream head member for sensing pressure differentials on the body portion downstream from the head member, but does not teach the unique geometry that permits the same sensor to be used in flowmeters for a wide range of pipe diameters.
Flowmeters similar to that shown in the last mentioned patent also are disclosed and discussed in U.S. Pat. Nos. 4,085,614 and 4,088,020. Particular attention should be paid to the angular arrangement of the edges of the head member or upstream plate, as well as the transverse width of the plate in relation to the length of the sensor bar that is used. The width of the sensor bar represented by the dimension T in drawings of U.S. Pat. No. 3,972,232 changes with different pipe sizes as shown in Column 9 of that patent. This is also the case in U.S. Pat. No. 4,085,614 as disclosed in Column 9 of that patent.
While various typical cross sectional configurations are shown in the prior art the geometries of the cross sections of the prior bluff bodies do not provide for the use of a body having a sensor mounting section that remains substantially constant in its critical dimension so that the same sensor assembly can be utilized for flowmeters used on different line sizes.